Electrochemical sensor having a mediator compound

ABSTRACT

An electrochemical sensor, especially for gases, is provided having a mediator compound based on transition metal salts of polybasic acids and/or transition metal salts of polyhydroxycarboxylic acids. The electrochemical sensor also contains a DLC, BDD or a precious metal thin-layer measuring electrode ( 3 ). The electrochemical sensor may be used for determining SO 2  and H 2 S.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 ofGerman Patent Application DE 10 2006 014 714.6 filed Mar. 30, 2006, theentire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention pertains to an electrochemical sensor, especiallyfor gases, having a mediator compound based on transition metal salts ofpolybasic acids and/or transition metal salts of polyhydroxycarboxylicacids and to the use thereof.

BACKGROUND OF THE INVENTION

Electrochemical measuring cells are widely used in the analysis ofsubstances, where potentiometry, voltammetry/polarography, coulometryand conductometry can be mentioned as the most important principles ofmeasurement. The use of electrochemical measuring cells for the analysisof gases has also been known for a long time. However, efforts are madeto develop new, more sensitive and more reliable sensors, especiallywhen toxic gases in the parts per billion (ppb) range are to be detectedand optionally even quantitatively determined.

Such electrochemical gas sensors are required to meet the followingrequirements for highly sensitive analysis:

-   -   Low residual current I₀;    -   no or at most only very weak effect of variations on the        humidity and/or temperature of the air on the residual current        I₀;    -   low cross sensitivity to interfering gases;    -   low double layer capacity of the measuring electrode, especially        in relation to dynamic measurement methods; and    -   high long-term stability.

The properties of an electrochemical gas sensor are decisivelydetermined by the material, the morphology and the layer thickness ofthe measuring electrode.

Platinum, gold or graphite are mentioned as measuring electrodematerials, e.g., in U.S. Pat. No. 3,795,589. Very many gases can bereacted directly, i.e., without a mediator, on the catalytically highlyactive precious metals platinum and gold. Therefore, it is frequentlyimpossible to reach the desired selectivity. The long-term stability ofthe catalytically less active graphite electrodes is slow and theseshow, depending on the electrode potential, high cross sensitivity to NOand NO₂.

DE 199 39 011 C1 discloses a sensor, whose measuring electrode consistsof diamond-like carbon (DLC). DE 101 44 862 A1 pertains to a measuringelectrode made of boron-doped diamond (BDD). These electrode materialspossess the properties required above, but they require a mediator,which selectively reacts with the analyte.

The use of ferroin (1,10-phenanthroline iron(II) sulfate) as a mediatoris proposed in U.S. Pat. No. 3,795,589. However, to make reaction withSO₂ possible, ferroin must be oxidized in the sensor to oxidation stateIII. This means long running-in periods and high residual currents(“cross-talk” with the auxiliary electrode), which are to be avoided.

SUMMARY OF THE INVENTION

The object of the present invention is to develop a mediator havinglong-term stability, which is especially selective for sulfur-containingoxidizable gases, especially SO₂. Besides the requirements mentioned inthe introduction, it is, furthermore, the object of the presentinvention to provide a gas sensor with reduced cross sensitivity tointerfering gases, short response time and high sensitivity to theanalyte. The analyte gas may be present both mixed with other gases ordissolved in a liquid, especially in water.

The subject of the present invention is an electrochemical gas sensor,which contains a novel mediator based on transition metal salts ofpolybasic acids and/or transition metal salts of polyhydroxycarboxylicacids, besides electrolytes, which are known per se.

The mediator compounds are specifically compounds that contain, besidesat least one acid group, at least one other group selected from amonghydroxyl and acid groups. In particular, the mediator compound is acarboxylic acid salt having, besides a carboxylic acid group, at leastone hydroxyl group, preferably at least two hydroxyl groups, and/or atleast one other carboxylic acid group. Tetraborates, such as sodiumtetraborate or lithium tetraborate, are also suitable compounds.

Transition metal salts, especially Cu salts of such mediators, permitthe selective determination of SO₂. However, such mediator compounds canalso be used to determine the concentrations of other target gases,e.g., H₂S.

It was surprisingly found that the Fe³⁺ salts, such as iron hydrogenphthalate and iron phthalate, are especially suitable among the mediatorcompounds according to the present invention for the determination ofH₂S. Formation of elemental sulfur was not observed. Contrary tocommercially available sensors, such sensors also do not show crosssensitivity to SO₂.

The mediators according to the present invention possess, furthermore,pH-buffering properties, so that the sensors can be exposed to gas forseveral hours without loss of sensitivity.

The corresponding Cu²⁺ salts are preferably used to detect or determineSO₂.

The mediators are preferably poorly soluble in the liquid gas sensorcomposition. The use of suspensions or solutions of the mediator withexcess solid offers a number of other advantages, such as:

-   -   Constant mediator concentration with variable air humidity;    -   identical equilibrium potentials at the measuring electrode and        the reference electrode if the reference electrode likewise        consists of carbon;    -   filter action of the excess solid; and    -   the sensor can be operated under anaerobic conditions if the        reference electrode likewise consists of carbon or the mediator        determines the potential thereof.

Hygroscopic alkali or alkaline earth metal halides, preferablychlorides, are preferably used as conductive electrolytes in an aqueoussolution. It is also possible to use, e.g., ammonium halides in case ofthe use of organic solvents, e.g., ethylene carbonate and/or propylenecarbonate.

Preferred are measuring electrodes from diamond-like carbon (DLC),especially those known from DE 101 44 862 A1, or measuring electrodesfrom boron-doped diamond (BDD), whose electrode material makes possiblean even larger potential window than DLC electrodes and can also be usedin case of extreme requirements, e.g., when determining an analyte withextremely high oxidation potential or very low reduction potential. DLC,which can be manufactured in a simple manner and cost-effectively, issufficient for many analytes.

The disclosure of DE 199 39 011 C1 and corresponding U.S. Pat. No.6,607,642, in which measuring electrodes made of diamond-like carbon,and of DE 101 44 862 A1 and corresponding U.S. Pat. No. 6,584,827, inwhich a measuring electrode made of boron-doped diamond (BDD) isdisclosed, are also made hereby expressly part of the disclosure of thepresent invention by reference (hereby incorporated by reference intheir entirety) especially concerning the design of the gas sensor andespecially concerning the measuring electrode.

It is suspected that the BDD and DLC electrode material in combinationwith redox-inactive electrolytes always requires an analyte that iscapable of effecting an “outer sphere electron transfer” in contact withthe electrode. Since only a small number of target gases accomplish sucha charge transfer between the electrode and the target gas according tothe experience currently available, it is necessary to add a mediator,which mediates a reaction on the measuring electrode, to the electrolytesolution.

The additional presence of a mediator offers the possibility of makingavailable a sensor that is highly selective against the desired analytegas by selecting suitable mediators.

In measuring electrodes made of DLC, diamond-like carbon is applied in avery thin layer to a gas-permeable membrane. The diamond-like carbonlayer may be produced according to a radiofrequency magnetron sputteringmethod or by means of other coating methods as well. The thickness ofthe layer of diamond-like carbon is 50 nm to 1,000 nm.

In case of the design as a BDD electrode, the measuring electrode isapplied as a thin layer of boron- or nitrogen-doped diamond to a poroussubstrate, the porous substrate preferably consisting of a nonwovenmaterial consisting of chemically pure quartz. If the measuringelectrode is formed on a porous carrier material, a separategas-permeable membrane before the measuring electrode may be eliminated.

The thickness of the thin layer of doped diamond is 0.5 μm to 5 μm. Incase of a layer consisting of boron-doped diamond, the doping consistsof 10¹⁹ to 10²¹ boron atoms per cubic centimeter of diamond. Fornitrogen, the doping is approximately 10²⁰ nitrogen atoms per cubiccentimeter of diamond.

Besides measuring electrodes consisting of DLC, BDD or precious metal(precious metal sputtered electrode), so-called carbon nanotubes (CNT)are also suitable for use as a measuring electrode material.

Carbon nanotubes are cylindrical carbon molecules from the family of thefullerenes, as they appear, for example, from EP 1 591 417 A1.

Measuring electrodes prepared from carbon nanotubes (CNT) have long-termstability, can be integrated in existing sensor constructions in asimple manner, are suitable for many mediators, and can be purchased atlow cost. There are only a small number of cross sensitivities caused bythe electrode material. This applies especially to multiwall carbonnanotubes (MW CNT). Such measuring electrodes are wetted by theelectrolyte solution over their entire surface, as a result of which alarge surface is obtained for the electrochemical reaction.

Carbon nanotubes have a structural relationship to the fullerenes, whichcan be prepared, e.g., by evaporating carbon according to a laserevaporation method. A single-wall carbon nanotube has, for example, adiameter of one nm and a length of about 1,000 nm. Besides single-wallcarbon nanotubes, there also are double-wall carbon nanotubes (DW CNT)and structures with a plurality of walls (MW CNT).

Carbon nanotubes are provided, due to their production, with metalatoms, e.g., Fe, Ni, Co, including the oxides thereof, so that suchcarbon nanotubes possess catalytic activities on measuring electrodes.It proved to be advantageous to remove these metal particles by acidtreatment.

However, it is possible to bind catalysts or mediators (e.g., porphyrinsor phthalocyanines) specifically to the carbon nanotubes. However, it isgenerally preferable to add a soluble mediator to the electrolyte.

The carbon nanotubes are advantageously applied to a porous carrier, anonwoven or a diffusion membrane. The carbon nanotubes are put togetherby self-aggregation or with a binder. Polytetrafluoroethylene (PTFE)powder is preferably used as the binder.

It is especially advantageous to prepare the carbon nanotubes from aprefabricated film, a so-called “buckypaper.” The measuring electrodecan then be punched out directly from the buckypaper. Large numbers canthus be produced at low cost.

The layer thickness of the carbon nanotubes on the measuring electrodedepends on the structure of the measuring electrode.

If the carbon nanotubes are in the form of multiwall carbon nanotubes,the layer thickness is between one μm and 1,000 μm and preferablybetween 50 μm and 150 μm. The layer thickness is between 0.5 μm and 500μm and preferably between 10 μm and 50 μm in the case of single-wallcarbon nanotubes.

The layer thickness also depends on the purity of the material. Thelayer thickness is rather at the lower end of the range in case ofespecially pure material.

The layer thickness of precious metal thin-layer electrodes, which areusually prepared according to a sputtering process, is between 100 nmand 500 nm. The catalytic activity of precious metal thin-layerelectrodes is much lower than that of the corresponding thick-layerelectrodes, but higher than in DLC or BDD electrodes. The preferredlayer thickness of precious metal thick-layer electrodes is between 200μm and 500 μm.

Classical gas diffusion electrodes (thick layer) are less preferred,because they have a high residual current and low selectivities.

The measuring cell contains the measuring electrode and the auxiliaryelectrode as well as preferably also a protective electrode and areference electrode. The sample contains the electrolyte solution andthe redox mediator in the dissolved form and optionally also as anexcess solid. The measuring cell has openings, which are provided with amembrane permeable to the analyte and otherwise close the measuring cellto the outside. The electrochemical cell contains a measuring electrode,protective electrode, reference electrode and the auxiliary electrode,which may be arranged in a coplanar, plane-parallel or radialarrangement in relation to one another and are flat. The gap between theplane-parallel electrodes may be filled with a separator, which ispermeable to the liquid medium and spaces the electrodes apart.

The mode of operation of the measuring cell is as follows: When analytegas is admitted to the membrane, whether the analyte gas is gaseous ordissolved in a medium, the analyte gas diffuses through the membraneinto the electrolyte and is oxidized or reduced by the mediator. Thetransition metal reduced or oxidized in the process is re-oxidized orre-reduced at the measuring electrode.

The most important processes that take place in the area of themeasuring electrode shall be briefly explained below on the basis of theexample of Cu²⁺ ions as a component of the mediator and of the analytegas SO₂. The SO₂ diffusing into the measuring cell from the outside isfirst oxidized by Cu²⁺, into SO₄ ²⁻:SO₂+2H₂O+2Cu²⁺⇄SO₄ ²⁻+2Cu⁺+4H⁺.The resulting Cu⁺ ions are re-oxidized at the measuring electrode:2Cu⁺⇄2Cu²⁺+2e ⁻

The electrolyte-mediator mixture according to the present invention canbe prepared as follows: So much CuCl₂ is added to an LiCl solution thata 0.2-1.0-molar and preferably 0.5-molar CuCl₂ will be formed. Thesensor has high sensitivity to SO₂ with this mediator. However, it has across sensitivity to H₂S and elemental sulfur is formed, which leads toclogging of the membrane during prolonged exposure to the gas.

The resulting chloro complex can then be mixed, e.g., with potassiumhydrogen phthalate, sodium tetraborate or trisodium citrate. Theresulting concentration should preferably agree with the above CuCl₂concentration and be especially about 0.5-molar.

A bluish-green precipitate is formed upon the addition of potassiumhydrogen phthalate or sodium tetraborate. Copper hydrogen phthalate,copper phthalate and copper tetraborate were described in the literatureas dimeric and polymeric compounds. These substances have not yet beenused as mediators so far. This also applies to the copper citratecompound, which is likewise available.

Due to the addition of potassium hydrogen phthalate, sodium tetraborateor trisodium citrate, it was possible to markedly reduce the crosssensitivity to H₂S, surprisingly to completely eliminate the formationof elemental sulfur, to markedly increase the sensitivity to SO₂ and tolower the residual currents.

An exemplary embodiment of the present invention is shown in the figureand will be explained in more detail below.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a longitudinal sectional view of a first electrochemicalsensor according to the invention; and

FIG. 2 is a longitudinal sectional view of a second electrochemicalsensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in particular, in the first embodiment of afirst electrochemical sensor 1, which is shown in FIG. 1, a measuringelectrode 3 is arranged in a sensor housing 2 behind a diffusionmembrane 4. A protective electrode 5, a reference electrode 6, a wick 7and an auxiliary electrode 8 are also arranged in the sensor housing 2 .The interior space of the sensor housing 2 is filled with anelectrolyte-mediator mixture 9. The mediator is additionally alsopresent as an excess solid 10. The measuring electrode 3, the protectiveelectrode 5, the reference electrode 6 and the auxiliary electrode 8 arekept at fixed distances from one another by means of liquid-permeablenonwovens 11, 12, 13, 14.

The gas enters through an opening 15 in the sensor housing 2. The firstelectrochemical sensor 1 is connected to a potentiostat, which is notshown more specifically, in the known manner.

FIG. 2 shows a second electrochemical sensor 20. Unlike in the firstelectrochemical sensor 1 according to FIG. 1, the second electrochemicalsensor 20 does not have reference electrode 6 as shown in FIG. 1 andinstead a disk-shaped reference electrode 16 is arranged behind theprotective electrode 5. The second electrochemical sensor 20 otherwisehas similar features. The interior space of the sensor housing 2 isfilled with an electrolyte-mediator mixture 9. The mediator isadditionally also present as an excess solid 10. Identical componentsare designated by the same reference numbers as in FIG. 1.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

1. An electrochemical gas sensor for detecting an analyte, theelectrochemical gas sensor comprising: a measuring electrode; anauxiliary electrode; and an electrolyte solution containing the mediatorcompound, said mediator compound comprising a transition metal salt ofan acid compound, said acid compound containing one of at least two acidgroups or at least one hydroxyl group and at least one acid group, saidmeasuring electrode detecting a concentration of one of H₂S and SO₂ in agas, wherein said electrolyte solution is in a saturated state, saidmediator being provided as an excess solid in said electrolyte solution.2. An electrochemical gas sensor in accordance with claim 1, whereinsaid acid compound is a carboxylic acid.
 3. An electrochemical gassensor in accordance with claim 1, wherein the carboxylic acid is anaromatic carboxylic acid with two or three carboxyl groups.
 4. Anelectrochemical gas sensor in accordance with claim 1, wherein thecarboxylic acid is phthalic acid, isophthalic acid or terephthalic acid.5. An electrochemical gas sensor in accordance with at least claim 1,wherein the acid compound is an aliphatic polycarboxylic acid.
 6. Anelectrochemical gas sensor in accordance with at least claim 1, whereinthe acid compound is citric acid.
 7. An electrochemical gas sensor inaccordance with claim 1, wherein the acid compound is gluconic acid. 8.An electrochemical gas sensor in accordance with claim 1, wherein theacid compound is a boric acid.
 9. An electrochemical gas sensor inaccordance with claim 1, wherein the electrolyte solution containsalkali or alkaline earth metal salts.
 10. An electrochemical gas sensorin accordance with claim 1, wherein the electrolyte solution containsLiCl.
 11. An electrochemical gas sensor in accordance with at leastclaim 1, wherein water or organic solvents are used as a solvent.
 12. Anelectrochemical gas sensor in accordance with at least claim 1, whereinethylene and/or propylene carbonate are used as a solvent.
 13. Anelectrochemical gas sensor in accordance with at least claim 1, whereinthe measuring electrode is a diamond-like carbon, boron-doped diamond, aprecious metal thin-layer electrode or carbon nanotubes.
 14. Anelectrochemical gas sensor in accordance with claim 9, wherein thethickness of the precious metal thin-layer electrode is 100-500 nm. 15.An electrochemical gas sensor in accordance with at least claim 1,wherein the transition metal salt is a copper salt or a Cu²⁺ salt. 16.An electrochemical gas sensor in accordance with claim 11, wherein theCu²⁺ salt is CuCl₂ and the concentration of CuCl₂ is between 0.2 mol and1.0 mol in a 2-10-molar LiCl solution.
 17. An electrochemical gas sensorin accordance with claim 11, wherein the Cu²⁺ salt is CuCl₂ and theconcentration of CuCl₂ is about 0.5 mol in a 2-10-molar LiCl solution.18. An electrochemical gas sensor in accordance with claim 1, whereinthe transition metal salt is an iron salt or an Fe³⁺ salt.
 19. Anelectrochemical gas sensor in accordance with claim 1, furthercomprising a gas-permeable membrane, said housing having an opening withsaid gas-permeable membrane wherein the analyte substance enters thearea of said measuring electrode via said gas-permeable membrane.
 20. Anelectrochemical gas sensor in accordance with claim 1, furthercomprising a reference electrode.
 21. An electrochemical gas sensor inaccordance with claim 1, further comprising a protective electrodearranged behind said measuring electrode.
 22. A method ofelectrochemical gas sensing for detecting an analyte, the methodcomprising: providing a measuring electrode; providing an auxiliaryelectrode; providing an electrolyte solution containing the mediatorcompound, said mediator compound comprising a transition metal salt ofan acid compound, said acid compound containing one of at least two acidgroups or at least one hydroxyl group and at least one acid group;positioning said measuring electrode and said auxiliary electrode insaid electrolyte solution containing the mediator compound to provide anelectrochemical gas sensor; and determining SO₂ concentration or H₂Sconcentration in a gas using said measuring electrode, said electrolytebeing in a saturated state, said mediator being provided as an excesssolid in said electrolyte solution.
 23. A method according to claim 22,wherein said electrolyte is or contains a chloride, said mediator beingformed from one of potassium hydrogen phthalate, sodium tetraborate andtrisodium citrate.
 24. An electrochemical gas sensor in accordance withclaim 1, wherein said mediator being formed from one of potassiumhydrogen phthalate, sodium tetraborate and trisodium citrate.